In 1989, Liu Xiaobo joined the peaceful protest movement centred on Tiananmen Square, and his life changed for ever. On 4 June, the Chinese government violently put down the demonstrations, imposing martial law and massacring hundreds of citizens. Two days later, Liu was arrested and imprisoned for 20 months. He was already known to the communist authorities as a dissident cultural critic, but now his books were banned and he was forbidden to teach.

Since then, Liu has become the most outspoken critic of the regime. He has been imprisoned on three further occasions, and is currently serving an 11-year sentence for helping to write Charter 08, a manifesto calling for democracy, free speech and basic human rights in China. His private response to the massacre has been to commemorate it every year with a poem. June Fourth Elegies makes public the first 20 poems in this sequence.

In his afterword, the translator Jeffrey Yang warns the reader that Liu is not a "wildly imaginative innovator" and that we should not expect too much linguistic nuance or technical discipline. This seems a little unfair. Admittedly, Liu's poetry is always at the service of his activism, but his obsession with questions of responsibility feeds directly into his poetic concerns: faced with a brutal regime, how should we form and style our response? What should we say, and not say?

The stakes are high, and Liu's introduction is highly critical of writers who pose as rebels but avoid direct engagement with politics: "They've matured without experiencing innocence; they've given up without experiencing the pursuit." In one poem, Liu sarcastically asks whether he should "pretend / to be Wang Wei or Tao Yuanming writing verse by a little stream ..." In the aftermath of the massacre, the most pressing choice facing Chinese intellectuals was whether to flee China or stay. Liu's poems depict both options as intolerable compromises: those who fled did so "to continue their grand meals of public opinion and fundraising", while those who stayed "had no time to clean the traces of blood from their turn-ups / but plunged head-first into the vast seas of trade".

Liu's decision to stay in China may have been an attempt to atone for a confession he wrote while in Qincheng prison. He is quick to condemn himself for this: "I betrayed the blood of the departed souls." In staying, Liu became an unblinking observer of both the inherent absurdity of totalitarian politics ("the Communist Youth League celebrates its 80th birthday"), and its pettiness ("At Tiananmen Square an armed policeman on / watch used his leather boot to kick and break / apart a child's snowman piled up high").

A powerful sense of history gives further depth to his observations. Liu refers repeatedly to the tomb of Qin Shi Huang, the first emperor of China. When Qin died, he had hundreds of soldiers in his Terracotta Army buried alive with him, so that he could rule another empire in the afterlife. This gives Liu a historical emblem of suicidal subservience before tyranny: "Our endless history depends / on the tombs of the emperors to demonstrate glory".

Blending activism and poetry, Liu has made contravening government policy an aesthetic principle. China forbids its citizens to commemorate the massacre: Liu responds by memorialising the event with a new poem each year. China denies the existence of Aids, and later the Sars virus: Liu repeatedly uses virus imagery in his work. He likens the anniversary of the massacre to "a fatal virus / no one's willing to approach", compares the culture of political subservience in China to Aids, and writes of "Sars politics / a nation is unable to breathe freely".

Yang refers to Liu's "bold, in-your-face directness", and this comes through in his surreal, visceral imagery: "In order to escape the abortion / the baby inside mother's belly / learns how to commit suicide". However, the tone of Yang's translation is often uncertain rather than bold. In order to translate the work into idiomatic English, Yang would have needed to take more liberties, but he was no doubt unwilling to do this, since he was unable to consult with the imprisoned Liu, or with Liu's wife Liu Xia, who is under strict house arrest.

Yang says that he found translating in isolation "painful" and "unnerving", but his understandable decision to convey Liu's meaning as literally as possible leads to some stilted phrasing. For example: "our disease is so ancient, exceeds / Christ's birth by a great temporal distance". It is not clear whether Liu wrote these lines in deliberately unidiomatic Chinese, or if something has been lost in translation. Yang also follows Liu's practice of using minimal punctuation: no full stops, and just a comma here or there when absolutely necessary. But in English, this has the effect of muting the anger in the voice.

When Liu won the Nobel peace prize in October 2010, he was not permitted to attend the ceremony. In his absence, the image of his empty chair became a powerful symbol of Chinese repression. There is something fitting in this, as Liu's poetry challenges itself to make an absent history present, and to give a voice to the politically silenced. As he says in the dedication to one poem: "The living should really shut their mouths and listen to the graves speak."